Python: while loop within a while loop

Question

I'm wondering why this code does not work.

loop = -10
loop2 = -10
while loop <= 10:
 while loop2 <= 10:
 if current_block:
 block = turtle.Turtle()
 block.shape("square")
 block.color("white")
 block.shapesize(stretch_wid=0.85, stretch_len=0.85)
 block.penup()
 block.goto(loop*20, loop2*20)
 loop2 += 1
 loop += 1

What I want to do is to create a 20x20 grid of squares centered at (0,0). Right now, only a line of squares are created at x-200

• Bill: Why not use for loops?
• Rufus: Not sure. Is for loops better in this case? What's the difference?
• Bill: For loops are best when you know how many times you will be iterating in each loop (e.g. 10). While loops are best for indefinite loops where you don't know in advance how many iterations there will be.
• Bill: For example, you could replace your while loops above with for loop in range(-10,11):. Then you don't need the loop += 1 statements.
• Rufus: Oh, nice! Thanks, I'll do that!

Answer 1

The loop2 variable retains its value, so the inner loop is not executed after the first iteration of the outer loop. You need to reinitialize loop2 in every iteration of the outer loop:

loop = -10
while loop <= 10:
 loop2 = -10 # Here!
 while loop2 <= 10:
 if current_block:
 block = turtle.Turtle()
 block.shape("square")
 block.color("white")
 block.shapesize(stretch_wid=0.85, stretch_len=0.85)
 block.penup()
 block.goto(loop*20, loop2*20)
 loop2 += 1
 loop += 1

• Rufus: Wow, that's some amazing response time! Thanks, that's exactly what I needed. :)